Ejection force modeling for stereolithography injection molding tools

Stereolithography (SL) molds have proven effective for short injection molding runs. However, they are susceptible to failure because of their poor mechanical properties, especially at elevated temperatures. A majority of these failures occur during the ejection stage, as a result of excessive ejection forces. An ejection force model was developed by combining the effects of thermal shrinkage and mechanical interlocking due to stair‐steps on the surface of SL tools. Finite element analyses were performed to validate and complement the ejection force equation. Measured forces and temperatures from injection molding experiments indicated that the ejection force model is valid for SL molds of both circular and non‐circular shape. The average differences between measured and predicted ejection forces were approximately 10%.     

Failure mechanisms in stereolithography injection molding tooling

Stereolithography tooling is a form of rapid tooling that has been used to injection mold limited runs of prototype parts. However, the process is not well understood and tooling life for fine mold features is difficult to predict. Injection molding processing conditions and feature geometry affect the number of parts that can be made before a mold fails. To study the effects of feature geometry, general purpose polystyrene parts were injection molded in molds made of DSM Somos 7110 stereolithography resin. The ACES build style was used, and no polishing was performed on the mold. The experimental results were compared with theoretical models developed for the two failure mechanisms for raised features in a stereolithography mold—failures during injection due to the flow pressure of the injected polymer; and failures during ejection, whereby the part pulled out a feature of the mold. Injection failures occurred in taller mold features due to the force of flow and the feature's geometry. Ejection failures occurred in the shorter features when the stress from the ejection force (distributed over the bond area) exceeded the yield strength of the mold material. Models were developed to predict the number of parts that a mold could make before mold features break off and were validated through experimental results.     

Numerical Molding Simulation for Rapid-Prototyped Injection Molds

Abstract:

Injection molding is a very mature technology, but the growth of layer-build, additive, manufacturing technologies (rapid prototyping, RP) has the potential of expanding injection molding into areas not commercially feasible with traditional molds and molding techniques. This integration of injection molding with rapid prototyping has undergone many demonstrations of potential. What is missing is the fundamental understanding of how the modifications to the mold material and RP manufacturing process impact both the mold design and the injection molding process. In addition, numerical simulation techniques have now become helpful tools of mold designers and process engineers for traditional injection molding. But all current simulation packages for conventional injection molding are no longer applicable to this new type of injection molds, mainly because the property of the mold material changes greatly. In this paper, an approach to accomplish numerical simulation of injection molding into rapid-prototyped molds is established and a corresponding simulation system is developed. For verification, an experiment is also been carried out with an RP fabricated SL mold. Stereolithography (SL) is an original and typical rapid-prototyping method, which is chosen as the study object in the paper.     

Draft angle and surface roughness effects on stereolithography molds

Stereolithography (SL) is a rapid prototyping process, which allows one to build complex shapes quickly. Current research investigates the possibilities of using this process to make injection molds. This would allow designers to manufacture and test molds easily and rapidly. One of the main issues with this technique is the effects of its surface on the part. Molds built by SL have high roughness. This gives rise to a high friction force between the part and the mold, and increases the ejection force needed to eject the part from the mold. High ejection forces often lead to damage or breakage of the part and the mold. Research was undertaken on the effects of draft angle and roughness on ejection forces. It was found that increasing the draft angle does not necessary assist the ejection of the part. As the draft angle increases, the roughness and hence the friction force between the part and the mold also increase. There is a trade‐off between draft angle and roughness. A model based on Glanvill's equation was developed to predict ejection force and was consistent with experimental results.     

Micro‐injection molding of CNT nanocomposites obtained via compounding process

Many research efforts have gone in the production of carbon nanotubes (CNT) composites for functional and structural applications and many processing methodologies have been experimented. Twin‐screw extrusion appears to be the most suitable way from the perspective of production scale up and commercialization of these composites. At the same time, micro‐injection molding process is considered as the key manufacturing technology for the mass production of miniaturized components and devices. Despite the massive literature about nanocomposites and microinjection molding process, few articles focus on the interaction between the compounding process and the following micro‐injection molding transformation processes. This article aims at analyzing the influence of the screw configuration used in compounding process on the rheological and technological properties of the resulting nanocomposites. Two different combinations of screw elements have been tested to incorporate CNTs in two different resins: LCP (liquid crystal polymer) and POM (polyoxymethylene) typically used in micro‐injection molding. The effects of the process set up have been observed studying first the rheology and then the moldability of nano‐compounds microinjected ribs with high aspect ratio. The nanofiller dispersion has been evaluated via light and transmission electron microscopy. The results confirm that, the screws show different capacity at promoting the dispersion of the nanofiller, which affects the moldability of micro‐injected CNT nanocomposites. The viscosity of the polymer seems a critical factor as well, because it influences first the dispersion of CNT bundle during extrusion and then the injection moldability of the composites in the micro‐channels. POLYM. COMPOS., 38:349–362, 2017. © 2015 Society of Plastics Engineers     

Photopolymerization kinetics of an epoxy‐based resin for stereolithography

Curing reactions of photoactivated epoxy resins are assuming an increasing relevance in many industrial applications, such as coatings, printing, and adhesives. Besides these processes, stereolithography (SL) makes use of photoactivated resins in a laser‐induced polymerization for 3D building. The kinetic behavior of photocuring is a key point for full comprehension of the cure conditions occurring in the small zone irradiated by the laser beam during the building process. Furthermore, the kinetic analysis is very important to determine the cure time needed for part building in a stereolithographic equipment. The mechanisms involved in a cationic photopolymerization are complex compared with radical photopolymerization. In this paper the photoinitiated polymerization of a commercially available epoxy‐based resin for stereolithography (SL5170) was studied by means of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). Substantial information about the SL5170 chemical composition and curing mechanism was determined through FTIR analysis. The polymerization rate and the maximum degree of reaction were determined directly from experimental DSC curves. Kinetic characterization of epoxy photopolymerization was carried out as a function of the temperature and irradiation intensity and experimental results were compared with an original mathematical model. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3484–3491, 2004     

Hot embossing of discrete microparts

The hot embossing process has so far been developed mainly for replication of surface structures on thermoplastic substrates. Because of the lack of a through‐thickness action, fabrication of discrete microparts such as microgears is considered difficult. In this study, embossing molds having multiple microcavities were used in a through‐thickness embossing process with a rubber‐assisted ejection mechanism. Microparts made of HDPE and ABS with each part weighing approximately 1 and 1.4 mg, respectively, were produced. When in the mold, embossed microparts were intermittently connected to each other through thin residual films of a thickness approximately 20 μm. The residual films were detached from the microparts during a rubber‐assisted ejection stage. Because no resin delivery paths, e.g., runners and gates, are needed for microcavities on the multicavity embossing mold, this micropart fabrication process could replace micro injection molding in many applications. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

快速成型工艺所用光敏树脂

立体光刻工艺是目前快速成型技术领域中研究得最多、技术上最为成熟、成型的零件精度最高的方法。该工艺的核心技术之一是光敏树脂。介绍了此类光敏树脂的基本概况 :树脂组成、固化机理、类别 ,并探讨了其现状与发展趋势     

Ejection force in tubular injection moldings. Part I: Effect of processing conditions

The development and manufacture of injection molds for high quality technical parts are complex tasks involving the knowledge of the injection molding process and the material changes induced by processing. In the case of some specific shapes (boxes, tubular fittings), the shrinkage is partially restricted by the mold. The molding shrinks against the core, inserts or pins. Thus, upon ejection, it will be necessary to overcome the frictional forces resulting from the shrinkage. The knowledge of the ejection force is a useful contribution to optimizing the design of molds with these features, and to guaranteeing the structural integrity of the moldings. A study on the effect of conditions on the ejection force required for deep tubular moldings is described for the cases of three common thermoplastic polymers. The studies were based on tubular moldings (60 mm diameter, 146 mm length, and 2 mm thickness). The injection unit cell consisted of a 1 MN clamp force injection molding machine, thermal regulator, and material dryer. During processing, pressure, temperature and ejection force evolutions were recorded. The results show that the processing conditions noticeably influence the ejection force. Polym. Eng. Sci. 44:891–897, 2004. © 2004 Society of Plastics Engineers.     

Improving the crack resistance of bulk molding compounds and sheet molding compounds

Fiber-reinforced plastics exhibit two types of mechanical failure: gross fracture and microcracking. Gross fracture involves both matrix and fiber failures. Principal resistance to crack propagation derives from partial decoupling of fibers and then stressing, remove finite volumes of them to fracture. Classical concepts of fracture mechanics can be applied to such composites, though modifications of methodology to treat anisotropy and other special effects are required. Microcracking occurs principally in the matrix phase and usually accompanies cyclic fatigue, drop impact, bending, or rapid cooling from molding temperatures. It lowers composite stiffness, environmental resistance and may reduce strength. Matrix resins require high fracture toughness to minimize or eliminate microcracking. This paper discusses cracking in bulk molding compounds and sheet molding compounds, complex materials containing high percentages of glass fibers and calcium carbonate filler. Microcracking can be greatly reduced by tire addition of small amounts of a rubber to the polyester matrix. Various tests such as impact, bending, acoustic emission and crack propagation demonstrate the improved toughness properties which result. No sacrifice of original strength characteristics occurs, and markedly improved resistance to damage has been noted with rubber modified epoxy and polyester matrix resins.     

塑料成型加工实用技术讲座(第三讲) 注射成型工艺对制品质量的影响

介绍注射成型工艺对制品质量的影响。指出注塑工艺参数主要有温度、压力、时间三类。对不同的塑料原料、模具结构、工艺参数的选择各不相同，且各工艺参数对制品质量的影响也有较大区别。在塑料原材料、模具结构一定的情况下，注射成型工艺参数的选取、设定对制品质量的作用显著。并指出了注射成型工艺参数的合理选用原则以及如何用PVT关系曲线对工艺参数进行优化。     

Mold-filling studies for the injection molding of thermoplastic materials. Part II: The transient flow of plastic materials in the cavities of injection-molding dies

A practically-oriented computer model which computes the temperature, pressure, and velocity fields in a cavity during the mold filling portion of the injection molding process is described. The model is structured so that it can be used for cavities having non-simple shapes and for commonly used molding compounds with complicated viscosity, shear rate, temperature relationships. Predictions from the model are found to be in good agreement with results obtained from exact solutions to special cases. Model predictions in molding problems have been found to correctly describe trends such as an increase in the pressure required to fill molds as injection rate, shot temperature, and mold temperature decrease, and to be reasonably accurate when compared to data for plaque, disc, and telephone housing molds over a wide range of molding conditions. Some illustrative examples of the use of the model in solving real molding problems are provided.     

In‐line monitoring of the injection molding process by dielectric spectroscopy

In recent years, electrical techniques like microdielectrometry have increasingly been utilized for their ability to continuously monitor, in a nondestructive way, the advancement of the reaction of thermoset resins under cure. This paper discusses an extension of this technique for the “in‐situ” monitoring of the crystallization of thermoplastics applied during an injection molding process. Electric sensors were positioned at the walls of the mold cavity so that an analysis of the volume dielectric properties of material during the filling, the post‐filling, and the cooling steps could be carried out. Poly(vinylidene fluoride) was chosen for this study. A correlation between the evolution of the dielectric parameters and the succession of the steps in this process was undertaken. The dielectric response was sufficiently sensitive to identify the steps of the closing of the mold, filling, post‐filling, cooling, and ejection of the part. In addition, information concerning the crystallization phenomenon near the wall or in the middle of the sample was collected. The gradual filling of the cavity of the mold was also identified by dielectric measurements. The temperature dependence of dielectric properties of the sample was beneficial in evaluating the increase of the temperature of the mold with the succession of injection cycles. The influence of the packing pressure has been clearly identified and confirms the usefulness of the dielectric method as a probe for detecting the shrinkage of the part during the optimization phase of the machine parameters. The dielectric method detailed herein provides a new non‐invasive technique and could be applied to a closed‐loop control of the injection molding process.     

Effect of combining boron nitride with fluoroelastomer on the melt fracture of HDPE in extrusion blow molding

Boron nitride (BN) is a new polymer processing aid which not only eliminates surface melt fracture in the extrusion of molten polymers, but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values that depend on resin type and additive concentration. In this work, the influence of BN as a polymer processing additive is first examined in the extrusion blow molding of high‐density polyethylene (HDPE) resins in order to evaluate its usefulness and performance in operations other than continuous extrusion. The equipment used includes both a Battenfeld/Fisher 50‐mm extrusion blow molding machine and a parallel‐plate rheometer. Two types of HDPE, which are blended with boron nitride at various concentration levels, are tested accordingly. It is found that the degree of BN dispersion, characteristics of the HDPE resins, extrusion temperature, and induction time play an important role in eliminating melt fracture. Finally, the influence of combining BN with fluoroelastomer, as an enhanced and potentially better processing aid on the melt fracture of a third HDPE is examined. It is found that such a combination is a superior processing aid that allows extrusion blow molding at very high shear rates.     

Estimating the residual fatigue lifetimes of impact‐damaged composites using thermoelastic stress analysis

A new experimental method is presented for quantifying impact damage and estimating the remaining fatigue lifetime of impact damaged polymer matrix composite materials. The procedure is demonstrated using composites of glass fiber reinforced polyurethane produced by injection molding and structural reaction injection molding. Thermoelastic stress analysis (TSA) was used to quantify the stress concentration associated with impact‐damage in test samples of each composite. Following impact and TSA imaging, the samples were fatigued to failure over a range of stress amplitudes. The TSA‐derived stress concentration factors were used to determine a modified stress amplitude that collapsed the impact‐fatigue data onto a master stress‐life curve. This approach provides a quantitative measure of impact damage and a practical methodology for estimating the residual fatigue lifetime of impact; damaged composites.     

奥拓轿车前保险杠注塑模浇注系统的CAE分析

针对奥拓轿车前保险杠大型注塑模具,采用CAE分析软件进行了优化设计。重点对注塑原料、设备和工艺进行了选择,并建立了制品的三维几何模型(包括浇口、气道的开设以及网格的划分等),成功地完成了熔体动态充模CAE模拟分析.分析结果表明,热流道四点进浇、四根气道及四个气体入口的对称布置方案是最理想的设计方案,热流通技术和气体辅助成型技术在大型注塑模上具有极大的应用前景。     

Fatigue performance of injection‐molded short e‐glass fiber reinforced polyamide‐6,6. II. Effects of melt temperature and hold pressure

Tensile and fatigue properties of an injection molded short E‐glass fiber reinforced polyamide‐6,6 have been studied as a function of two key injection molding parameters, namely melt temperature and hold pressure. It was observed that tensile and fatigue strengths of specimens normal to the flow direction were lower than that in the flow direction, indicating inherent anisotropy caused by injection molding. Tensile and fatigue strengths of specimens with weld line were significantly lower than that without weld lines. For specimens in the flow direction, normal to the flow direction and with weld line, tensile strength and fatigue strength increased with increasing melt temperature as well as increasing hold pressure. The effect of specimen orientation on the tensile and fatigue strengths is explained in terms of the difference in fiber orientation and skin‐core morphology of the specimens. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers.     

Numerical and experimental studies on the ejection of injection‐molded plastic products

Numerical and experimental studies have been conducted on the ejection stage of plastics injection molding process. A numerical approach is proposed to predict the ejection force from the mold‐part constraining and friction forces as the product cools in the mold cavity up to the moment of ejection. The finite element thermoviscoelastic solidification analysis has taken into account the stress and volume relaxation behavior of polymers under the cavity‐constrained condition. The predicted ejection force and its distribution over ejector pins are validated by injection molding experiment of rectangular boxes using a polycarbonate resin. Different cases of the ejector pin layout are evaluated to examine the effect of the number, location and dimension of ejector pins, so as to identify the balanced layout causing minimum stress and deformation to the product. The approach is also applied to another product geometry which shows complex distribution of the mold‐part constraining and friction forces and involves multi‐step operations in the demolding stage.     

Investigation of the role of transverse flow in co-injection resin transfer molding

A co-injection resin transfer molding (CIRTM) process has been developed at the University of Delaware's Center for Composite Materials in collaboration with the U.S. Army Research Laboratory. It enables two or more resins to be simultaneously injected into a mold filled with a stationary fiber preform. This process allows for the manufacturing of co-cured multi-layer, multi-resin structures in a single processing step. A separation layer is used to provide resin compatibility during cure and to control resin mixing. Scaling issues relating the role of transverse permeability in resin mixing are investigated. This study presents two different approaches taken to understand the causes of transverse flow and to quantify the amount of transverse flow that occurs during processing. The first approach, a one-dimensional model, explains the important parameters that govern resin flow in CIRTM. The second approach is based on an existing finite element code that is modified to allow for the injection of multiple resins. The total amount of transverse flow was quantified using the finite element code. This research shows that the CIRTM process requires a totally impermeable separation layer if CIRTM is used to manufacture large parts and/or if the resins injected have significantly different viscosities.